The present invention relates to a particle measuring fluid cell employed in a system to measure the size of particles entrained in a liquid stream or gas flowing through the cell and, more particularly, to shims which define a narrow passageway for the liquid through the cell.
Devices for measuring the size of particles entrained in a liquid stream, wherein the devices employ a collimated beam of light through a collimating lens system and a light sensitive element, are known in the prior art. Such a device is disclosed in U.S. Pat. No. 4,260,258 in the name of Dennis H. Rose et al. In the device disclosed therein, a particle measuring fluid cell includes a light source in the form of a light emitting diode, which projects a beam of light through a collimating lens system, across the fluid stream contained in a fluid flow channel and onto a light sensitive element. Particles carried by the fluid stream cast a shadow on the light sensitive element, which causes a change in the output signal level from the light sensitive element. By utilizing sensitive signal detectors, the change in the signal can be correlated with a given particle size, so that particles of various particle sizes can be measured and counted. In the region where the light beam intersects the fluid stream, a very narrow channel for the fluid stream is defined by a pair of shims sandwiched between flat plate members.
A problem arises from the fact that such particle measuring fluid cells are used for measuring particles in a wide variety of liquids and gases, such as water, strong acids and gaseous hydrochloride acid, having different corrosive characteristics. For measuring particles in water, the shims are made of stainless steel, but when the liquid is a strong acid, the shims must be made of platinum or some other expensive metal not corrodible by the acid. Thus, different devices employing parts of different materials must be made and stocked so that all applications of the devices can be satisfied. The problem is complicated by the fact that the fluids flowing through the cells are under high pressure.
As was mentioned above with connection with U.S. Pat. No. 4,260,258, the stainless steel is suitable for the shims of particle measuring fluid cells where the liquid is water, but unsuitable for some acids, which will corrode stainless steel. Shims made entirely of non-metallic materials have also been found to be unsatisfactory. The fluid pressure inherent in particle measuring fluid cells requires the shims to be sandwiched between the flat plate members under considerable compressive force. When the plates are made entirely of a relatively soft material, such as a plastic, the material of the plates cold flows under the large compressive forces exerted by the flat plate members. This cold flowing leads to leakage in the fluid cell and blockage of the flow passage.